In most manufacturing processes, a variety of chemicals are used in a multiplicity of manufacturing steps. For instance, there are frequently several hundred manufacturing steps involved in the fabrication of an integrated circuit. The manufacturing steps may include etching, cleaning, deposition and various other necessary steps. A variety of chemicals, including liquid chemicals, are used in the processing steps either to etch a specific circuit on an IC chip, to clean after certain processing steps, to deposit layers from reactant chemicals, or to carry out any other necessary fabrication steps. An important consideration in using chemical liquids is the transporting/storage of the material. For instance, when a photoresist liquid is used in IC processing, the photosensitivity and the lifetime of the material depends on its storage temperature and its exposure to light. It is therefore important to maintain a photoresist liquid within a temperature range of between 5.degree. C. and 20.degree. C. and in a relatively dark environment.
In the transporting of IC processing liquids, plastic piping such as Teflon pipes are frequently used due to the fact that the liquid being transported is frequently volatile and flammable. The Teflon pipes are also ideal for chemical liquids that are highly corrosive. The use of plastic piping greatly reduces the risk of explosion or fire that are sometimes caused by sparks generated between metal pipes. When Teflon pipes are used, they are frequently connected together by Teflon couplings between various sections of the pipes. The pipes and the couplings are connected by male and female threads provided on the pipes and in the couplings respectively. Based on the low rigidity of plastic materials a connection between a Teflon pipe and a Teflon coupling can only be made by hand tightening the two parts together. Any tightening force larger than that provided by hand tightening would cause the stripping of threads on the Teflon pipes. A joint formed between a Teflon pipe and a Teflon coupling therefore is not extremely tight and consequently, would allow air to enter the pipe and forms bubbles or micro-bubbles. The micro-bubbles are bubbles formed on a microscopic scale and sometimes cannot be observed by human eyes.
An illustration of a conventional liquid supply system 10 is shown in FIG. 1. In system 10 as presently used in a semiconductor fabrication facility, a liquid reservoir or holding tank 12 is first provided for storing liquid 14. Liquid 14 can be fed to the tank 12 from a liquid inlet 18 through a shut-off valve 20. At the uppermost portion of the liquid reservoir 12, a vent outlet 24 is provided for venting the tank to the atmosphere. A filter 26 is provided in conduit 28 for the vent system such that only air and not liquid can be vented out the vent outlet 24. Conduit 32 allows the delivery of liquid 14 in tank 12 to a pump 34 that is driven by motor 36. The pump 34 is controlled by a host computer (not shown) of the process machine such that it dispenses liquid of a predetermined volume in a predetermined interval of time as instructed by the computer. The pump 34 can be suitably provided of the bellow-type which has a preset volume of displacement during each activation of the bellow such that a predetermined amount of liquid is delivered through conduit 40 to a liquid/air separating device, or filter 42. For instance, in a liquid supply system for a photoresist coating material, a bellow-type pump can be preset to deliver 8 cm.sup.3 of the photoresist upon activation of the bellow to coat a wafer.
The liquid/air separating device, or filter 42 can separate trapped air from the liquid when such a mixture is delivered to filter 42 through conduit 40. The trapped air after being separated out of the liquid cumulates at an upper portion of filter 42. The filter 42 is designed as a one-way filter such that back flow of liquid into conduit 40 is not possible. A liquid flow that is substantially without air bubbles can be pumped by the bellow pump 34 to enter conduit 46 through shut-off valve 48 for dispensing onto a wafer (not shown). Since conduits are normally made of a plastic material, such as Teflon, and therefore are translucent or semi-transparent, any trapped air bubbles in the liquid can be visually observed by the machine operator. When air bubbles are observed, the operator opens valve 52 and discharges a significant amount of the liquid into a waste collection tank 54 through conduit 56 in order to get rid of the air bubbles.
As previously discussed, since plastic piping can not be tightly joined together, air leaks into the conduit and forms trapped air bubbles in the liquid. In the conventional liquid supply system 10 shown in FIG. 1, each time air bubbles are observed in the liquid conveying conduit, a large quantity of liquid must be drained through conduit 56 into the waste collection tank 54 in order to purge out all the trapped air bubbles. This presents a significant source of waste of the process liquid used in a manufacturing process and adds significantly to the cost of fabrication since most process liquids used in IC fabrication are high cost materials.
It is therefore an object of the present invention to provide a method for supplying a liquid to a process machine that does not have the drawbacks and shortcomings of the conventional methods.
It is another object of the present invention to provide a method for supplying liquid to a process machine that is capable of supplying a liquid which is substantially without trapped air.
It is a further object of the present invention to provide a method for supplying a liquid to a process machine that is capable of recirculating a liquid containing air bubbles into a liquid reservoir such that the draining of the liquid is not necessary.
It is another further object of the present invention to provide a method for recirculating a process liquid such that a substantially air bubble-free liquid can be delivered to a process machine.
It Is yet another object of the present invention to provide a method for recirculating a liquid containing air bubbles that have leaked into the plastic pipes is sent back to a liquid reservoir for venting.
It is still another object of the present invention to provide a method for recirculating a liquid that is capable of removing air bubbles in the liquid while not wasting any of the liquid to a drain tank.
It is still another further object of the present invention to provide a recirculating liquid supply and recovery system that is capable of supplying an air bubble-free liquid to a process machine while recirculating a liquid flow containing air bubbles back into a liquid reservoir.